Polymerisable liquid crystal (LC) materials are commonly used for the preparation of optical films in liquid crystal displays. These materials usually contain a certain amount of compounds with two or more polymerisable groups (di- or multi-functional), which are crosslinked to give a hard film.
However, during polymerisation certain polymerisable materials, like for example acrylates, suffer polymer shrinkage [see R. A. M. Hikmet, B. H. Zwerver and D. J. Broer Polymer (1992), 33, 89]. This shrinkage places a lot of strain in the polymerised film and acts to reduce the adhesion between the film and the substrate.
One technique reported in prior art to overcome this problem concentrates on modifying the substrate to improve its adhesion to the polymerised film. For example, the substrate can be subjected to special treatment, for example flame treatment as disclosed in U.S. Pat. No. 2,795,820 or GB 0 788 365, corona treatment as reported in DE 195 80 301, or plasma treatment as reported in R. L. Bersin Adhesives Age (1972) 15, 37.
Alternatively, a separate adhesion or coupling layer (typically a solution of organo-silane materials) can be coated onto the substrate to help increase adhesion of a polymer film to a substrate, like e.g. the commercially available Addid 900® (from Wacker GmbH, Burghausen, Germany), an amino-functional trimethoxy silane.
U.S. Pat. No. 5,631,051 discloses a method of preparing an optical compensation sheet on a transparent substrate of triacetyl cellulose (TAC), by first providing an adhesion layer of gelatine on the TAC film. Then an aligning layer is formed by coating a solution of denaturated polyvinyl alcohol (PVA), which was chemically modified by addition of polymerizable groups, onto the gelatine layer, evaporating the solvent and rubbing the surface of the polymerised PVA layer unidirectionally, Finally an optically anisotropic layer comprising discotic LC material is coated onto the rubbed surface of the PVA layer and polymerised.
U.S. Pat. No. 5,747,121 discloses a method of preparing an optical compensation sheet by coating a solution of denaturated polyvinyl alcohol (PVA), which was chemically modified by addition of polymerizable groups, onto a transparent substrate, evaporating the solvent and rubbing the surface of the PVA layer unidirectionally. Then an optically anisotropic layer comprising discotic LC material is coated onto the rubbed surface of the PVA layer and polymerised. Afterwards the film is subjected to heat treatment whereby the PVA layer and the discotic LC layer are reported to be chemically bonded to each other via free, crosslinkable groups.
However, all of the above methods have a distinct disadvantage in that they involve an extra processing step. Furthermore, the use of separate adhesion or aligning layers comprising isotropic materials like gelatine or PVA can negatively influence the performance of the liquid crystal film when used e.g. as optical film.
To overcome this problem an adhesion promoter can be directly added to the polymerisable LC material formulation. Addid 900®, or a similar material, is a typical additive. However, when these materials are added to polymerisable LC mixtures it is often difficult to obtain well aligned films, such as those required for optical films.
In particular films or coatings of polyimide (PI), which are commonly used as alignment layers to induce uniform orientation of the LC material provided thereon, often have only low adhesion to the polymerised LC material.
Therefore, there is a need for an advantageous method to improve the adhesion of a film or coating, which is used as alignment layer for LCs, to an LC polymer film. The method should not negatively affect the optical and mechanical properties of the alignment layer, like its transmission and its stability against temperature, mechanical stress and solvents.
It was an aim of the present invention to provide such a method. Another aim of the present invention was to provide an alignment layer having improved adhesion whilst not affecting its optical and mechanical properties. Other aims are immediately evident to the expert from the following description.
The inventors of the present invention have found that the above mentioned drawbacks of prior art methods can be overcome by using an alignment layer that is obtainable from a precursor material comprising a low amount of a polymerisable mesogenic compound (reactive mesogen). This compound improves adhesion of the alignment layer, in particular when used as substrate or alignment layer for an LC polymer provided thereon, whilst not negatively affecting its optical and mechanical properties.